1. Field of the Invention
The invention relates to an exhaust gas control apparatus and exhaust gas control method for an internal combustion engine, which includes a purification mechanism that purifies exhaust gas discharged from an internal combustion engine.
2. Description of the Related Art
In general, an internal combustion engine is provided with an apparatus that purifies exhaust gas using a purification mechanism such as a catalyst. The purification mechanism is provided in an exhaust passage for the internal combustion engine. While the purification mechanism is used for a long time, particulate matter (PM) and sulfur components gradually adhere to the purification mechanism, and therefore a function of the purification mechanism tends to decrease. Accordingly, a recovery process is performed at given time intervals. In the recovery process, fuel is supplied to the purification mechanism, and PM and the sulfur components are burned and removed so that the function of the purification mechanism is recovered.
When the recovery process is performed, that is, when fuel is supplied to the purification mechanism, for example, the fuel is injected to the exhaust passage from a fuel supply valve provided in the exhaust passage at a predetermined pressure. In a case where the fuel is constantly injected from the fuel supply valve, the fuel may remain in the exhaust passage without being conveyed to the purification mechanism such as the catalyst, and the fuel may adhere to a wall surface of the exhaust passage during a period from an exhaust stroke of a cylinder until an exhaust stroke of another cylinder.
In order to solve such a problem, Japanese Patent Publication Application No. JP (A) 2002-106332 discloses an apparatus which supplies fuel during a period in which exhaust gas is discharged from a cylinder. In this apparatus, fuel is injected to an exhaust passage according to a crank angle phase of an internal combustion engine. In other words, the fuel is injected to the exhaust passage during a period in which an exhaust valve is opened, that is, during a period in which exhaust gas is discharged from the cylinder.
In order to efficiently recover the purification mechanism such as the catalyst, it is desirable that the fuel supplied to the exhaust passage should be appropriately conveyed to the purification mechanism by an exhaust gas flow. The exhaust gas flow in the exhaust passage is generally influenced by exhaust gas discharged from plural cylinders. Therefore, even when fuel injection timing is set such that the fuel is supplied during the period in which the exhaust valve in a specific cylinder is opened, the fuel supply timing does not necessarily become appropriate for a flowing condition of the exhaust gas in the exhaust gas passage. Particularly, this tendency becomes noticeable in a collecting exhaust passage into which exhaust gas discharged from each of plural cylinders flows.
In the aforementioned conventional apparatus, since the fuel supply timing is simply set such that the fuel is supplied during the period in which the exhaust valve is opened, the fuel can be injected intermittently. However, the apparatus needs to be improved, considering the fuel supply timing needs to be set such that the fuel is appropriately supplied to the purification mechanism while suppressing adhesion of the fuel to the exhaust passage.